- Straight – It is almost non-existent among natural rivers. Extremely short reaches of the river may be straight.
- Meandering – It a sinuous channel of river. A meander is formed when moving water in a river erodes the outer banks and widens its valley, and the inner part of the river has less energy and deposits silt.
- Braided – It is a channel that consists of a network of small channels separated by small and often temporary islands called braid bars. Braided channels occur in rivers with high slope and/or large sediment load.
- Anastomosing – Like braided channel branching of small channels from a single occurs at first, but after that separated channels again merge.
The controlling factor on river development is the amount of sediment that the river carries. Once a water way crosses a threshold value for sediment load, it will convert from a single channel meandering river to a braided channel (Leopold and Wolman, 1963). Bank erosion, however, occurs mainly in meandering rivers. In meandering rivers, river-channel migration takes place through erosion of the cut bank and deposition on the point bar. River-channel migration is the lateral motion of an alluvial river channel across its floodplain due to processes of erosion of and deposition on its banks and bars (Wikipedia definition). Meandering of a river is caused by nature but sometimes by human activities. Due to natural or human or both activities, most rivers in the world are subject to meandering along with bank erosion. But in view of devastation, the Mississippi-Missouri River System of North America, Ganges, Brahmaputra and Mekong Rivers of Asia, Amazon River of South America, and River Nile of Africa are most important among them.
3* shows the relationship between the river channel migration rate and the drainage area of the river (Hudson and Kesel, 2000*). However, the rate of channel migration is not same for all rivers with the same drainage area. This happens because the rate of migration also depends on the material that constitutes the river banks. This is exactly what happens in the case of Guadalupe River (a meandering river) in the United States, where the rate of erosion is less than expected (Gantt and Humberson, 2004).
Both Mississippi and Missouri Rivers have been facing meander migration. It is a process in which water flow erodes soil on one bank and deposits it on the opposite bank, i.e., a gradual shift of bank line occurs over time (Briaud et al., 2007). The Missouri River bank is eroding at a high rate on the Lower Brule Reservation central South Dakota in the United States. The Lower Brule Sioux Tribe (LBST) has estimated that the reservation is losing its shoreline in some locations at a rate of approximately 8 feet per year (Neitzert et al., 2012). The long-term impact of bank erosion on cultural and environmental losses is under investigation.
A great amount of channel migration of the Mississippi River has occurred over the last 200 years (Maynord and Martin, 1996). In the early 1800s, U.S. Army Corps of Engineers (USACE) began removing snags and dredging the main channel of the Mississippi River and later the main channel of the lower Missouri River was subject to river engineering like channel straightening, dike construction, etc. to increase the ease and safety of river navigation (Alexander et al., 2012). That effort made navigation easier and safer for the time being, but that human-induced channel migration caused substantial bank erosion. The Modification of Mississippi River is still continuing for better navigation. However, recognising the “devastating damage done by modifications to the Upper Mississippi River and the importance of wildlife and people living along the river”, the U.S. Congress authorized the Mississippi River Restoration Ecosystem Management Program in 1986 (Prairie Rivers Network, 2012).
The Illinois River is a major tributary of the Mississippi River in the United States. Most of the drainage area is situated in the State of Illinois. The river has been changing its course through bank erosion since the year 1939 mainly due to construction of locks and dams to facilitate the movement of river traffic (Bhowmik, 2008). Bank erosion has been occurring because of a number of natural forces and human induced activities. The analysis has shown that about 74 percent of the bank erosion sites were due to seepage flows. About 28 percent of the bank sections have shown the evidence of river traffic induced impacts. Also there are other factors like eddy currents, disturbed flows due to exposed tree roots, surface drainage, etc. that caused erosion.Laderoute and Bauer, 2013). An example is the lower Gordon River in Tasmania in Australia. The wakes generated by river traffic are the main driver of bank erosion, and there people have concerns for quality of the drinking water, loss of property, and loss of aquatic habitat. The wakes generated from large vessels in earlier decades were considered negligible or acceptable. But the effects have become noticeable after the introduction of high-speed craft (HSC) that are capable of carrying vehicles and passengers. The adverse effects of bank erosion by boats like large catamarans, which are used as industrial ferries, are mainly due to their high speeds and large size, producing longer wakes than conventional ships (Parnell et al., 2007). The lower Gordon River is a river being severely impacted by boat wake erosion. For this river there are regulations that permit a maximum wave height for any vessel passage. However, monitoring and experimental testing has demonstrated that most of these regulations are not very effective against erosion as recreational traffic is not being subject of these regulations although it has a disproportionally large impact (Bradbury, 2005).
Bech, 2006; Pearce, 2013). Early phases of river modifications protected the settlements against flood and provided new fields for agriculture, but it did not reduce the risk of bank erosion. Straitening of rivers caused bank erosion when a bank protection failure had occurred resulting in significant damages and financial losses. Moreover, the modification of river banks has threatened the ecology of rivers in many areas of Europe.
Length in km
|Before development||After development|
An example is the bank erosion of Danube River (Jones et al., 2007). It is the second longest river (2850 km) in Europe and an important transport route. Danube River is historically described as gravel dominated, laterally active river with elements of both braided and meandering channel types (SEDDON). Danube River basin supports important economic activities and so the banks of the river were modified to facilitate navigation (Table 1). But it did not work as perfectly as it was envisaged. In Hungary, rivers were modified and shortened for flood control and navigation. Now the Middle Danubian loess bluff is at danger of bank erosion, more than ten settlements are at risk and a large amount of municipal and industrial infrastructure has already been damaged (Szalai et al., 2013). Archaeological sites also are at risk along Bulgarian Danube bank because of erosion (Nikolova et al., 2012). Additional, Danube River is the border line between Serbia and Croatia. Serbian farmers lost their arable lands, which they occupied on the left river banks in Serbia 50 years ago, because of very intensive lateral erosion and vice versa for Croatian farmers (Dragićević et al., 2013*).
Now most of the countries in Europe realise the need for restoration of rivers to their natural course and application of bioengineering techniques for river bank erosion control (Evette et al., 2009; Donat, 1995). The idea is to allow natural sediment transport systems with acceptable and manageable erosion and deposition rates. The restoration of rivers, however, is not simple as they encompass cities, industries, hydroelectric dams, etc. For the need of restoration, the cities might have to move, hydroelectric plants would have to be closed, and industries would have to be relocated.Wong et al., 2007). The lateral erosion on the Nile river banks in Egypt is another example of river bank erosion. Nile river meandering and the associated processes of bank erosion and deposition accelerated with human activities. Erosion has its impact on both economy and environment. Bank erosion has caused decrease in agricultural lands which in turn has reduced the agricultural production (Ahmed and Fawzi, 2009). It has been recommended to protect the river bank from further movement and erosion. The recommendations are
- to weaken the secondary currents created by the river bends,
- to regularly monitor the river banks and islands and measure the rates of erosion and deposition, and
- to monitor sand bars and subsurface islands and to identify with flash lights to mitigate navigation problems.
MRC, 2010). The channel pattern of the Mekong is meandering with low sinuosity (Wood et al., 2008). The lower Mekong basin includes Laos, Thailand, Cambodia, and Vietnam. The river bank zone in the basin provides places for human settlement and also consumption goods and inputs to production (Miyazawa et al., 2008). Thus, bank erosion in Mekong River not only displaces population but also brings about loss of household income sources. It was reported that about 600 families in Tonpheung district of Bokeo province in Vietnam were forced to migrate from their homes because of river bank erosion over the past three years (Pongkhao, 2008). Villagers had been moving further inland, away from the Mekong River, to escape the problem. It was reported further that these people will be forced to move out again if the bank erosion process continues. Apart from the socioeconomic problem there is also a political problem. The political border between Laos and Thailand (about 1100 km long) is the deepest line (thalweg) of the Mekong river channel. But this political border has shifted due to excessive erosion of the Laos river bank. Moreover, the altered flow channel made an island that was once part of Laos is now part of Thailand (Brown, 1999). As it is a very sensitive issue, the two countries have agreed to settle the border problem.
The Yellow River (or Huang He) is the second largest river in China after the Yangtze River. The river meanders for 5000 km through nine provinces and a watershed of about 680 000 km2. The river was called “China’s Sorrow” because of frequent occurrences of flood along the river and sufferings of millions of people. The worst flood disaster in world history occurred in August 1931 along this river and killed an estimated 3.7 million people. The Sanmenxia Dam on the Yellow River was constructed in 1960 to prevent floods, provide water for irrigation, and produce hydroelectric power. However, significant silt loads in the Yellow River were not adequately considered in the planning stage. The reservoir water basin was largely filled with silt only four years after construction, and the reservoir was subsequently taken out of operation. In subsequent years three more dams were constructed: Liujiaxia Dam in 1968, Longyangxia Dam in 1985, and Xiaolangdi Dam in 1997. Two significant bank erosions were observed: one during 1961 to 1964 associated with completion of Sanmenxia Dam, and the other during 1998 to 2004 associated with completion of Xiaolangdi Dam (Ma et al., 2012).
Bangladesh, a riverine country, is suffering heavily from river bank erosion. Thousands of Bangladeshi population were forced to migrate from their place of origin due to bank erosion. The major rivers of Bangladesh are Padma River, Jamuna River and Meghna River. These three rivers have eroded several thousand hectares of floodplain, several Kilometers of roads and railways and have displaced people. The Ganges-Padma River in Bangladesh is a meandering river. But lately, the Ganges-Padma becomes a braided river because of high sediment transportation by Jamuna and deposition of Ganges-Padma river bed (Yeasmin and Islam, 2011). During 1970 – 2000, two major rivers of Bangladesh, Padma and the Jamuna eroded 180 000 hectares of land and about 200 000 people were displaced (Islam and Rashid, 2011). Displaced people experienced substantial socio-economic impoverishment and marginalisation because of forced migration and inequitable access to land and other resources (Mutton and Haque, 2004). Another erosion afflicted country in Asia is Myanmar. Erosion of river banks along the Irrawaddy and Chindwin rivers in Magwe Division, central Myanmar is common during the rainy season and riverine communities are frequently compelled to migrate (Mann, 2013). Some families moved several times during 1993 to 2012 and lost everything. In spite of government’s initiative little has been done to prevent further erosion.
India is also a land of rivers like other Asian countries. Seven major rivers along with their numerous tributaries make up the river system of India. Floods, which are recurrent phenomena in India, cause severe bank erosion. Two rivers that are subject to severe bank erosion are River Ganges and Brahmaputra River, a braided river (Sarma, 2013; Mili et al., 2013; Phukan et al., 2012*). So far, various steps like bank embankment have been taken up for flood control. But erosion along the embankments and natural banks of the river systems is still a serious problem. Particular mention could be made of the erosion problem of the embankment systems in some states like Assam, Bihar, West Bengal, etc. (India-WRIS). Since the embankments are under serious erosion by the major rivers and their tributaries, criticism about existing embankments have emerged. Now there are distinctly two opposite views like in other countries, too (Planning Commission, 2011).
One opinion is that the problems of flood can be solved by removal of all the existing embankments and allowing natural bank erosion of rivers. The other is that the flood/erosion problems can be solved by constructing more and more streches of embankments and also by raising and strengthening them.
2 shows percentage of continent-wise displaced population in the world due to all types of natural disasters in the years 2010 and 2011. Worst sufferer was Asia and least sufferer was Oceania. The displacement was caused mainly by hydrological disasters, which include floods, storms, and wet mass movements. In 2010, the contribution due to hydrological disasters was nearly 85 percent (IDMC, 2011*) and in 2011 it was more than 71 percent (IDMC, 2012*). Rest of the human displacement was due to geophysical (earthquake, volcanic eruptions, etc.), meteorological (storms), climatological (extreme temperature, etc.) and biological disasters (epidemics, etc.).
Percentage of total displaced population in the world
Percentage of displaced population relative to the total world population
|Year 2010||Year 2011||Year 2010||Year 2011|
One such study was the analysis of socio-economic consequences of the Kolubara river bank erosion in Serbia (Dragićević et al., 2013). The analysis was in terms of land loss, land use changes and economic loss. The study area in this analysis had economic importance, and there was significant density of the agricultural population and settlements. Because of bank erosion in the study area, the farmers who had arable land on the river bank lost their land assets by the river. The loss of corn production was 3255 tons and of wheat production was 1271 tons till the year 2010. The level of production losses was steadily increasing over time. It was estimated that the total value of the permanent losses of arable land was 80 560 USD, and the total loss in agricultural production was 634 240 USD till 2010.
Erosion of the reservoir of the Three Gorges Dam and downstream Yangtze River banks is a threat to human settlement and to one of the world’s biggest fisheries in the East China Sea. It is expected that more than four million people have been displaced due to construction of dam, downstream bank erosion and other environmental impacts (French, 2007). There is increasing concern that the people displaced due to construction projects and erosion face long-term risks of becoming poorer and are also threatened with landlessness, food insecurity, joblessness, and social marginalisation (Gleick, 2009).
The socio-economic impact becomes more severe when people are more vulnerable to natural disaster. This is what happens in Asia. High population density and poor economic condition of the people (Table 3) in Asia make consequences of natural disasters more devastating. A large section of population is bound to stay in high risk zones like banks of meandering rivers. This is a common scenario in almost all countries in Asia. In Bangladesh, the poor, small landowners who live near the river bank are the most affected victims of bank erosion. Bank erosion affects their well-being in terms of safety and shelter, as well as their sources of livelihood (Brouwer et al., 2007). Riverbank erosion is bringing about unemployment, landlessness and poverty in every year, and is increasing over time. It has been supposed to be responsible for the unstable condition in the country (Rahman, 2013). In Indonesia, those who suffer most during floods are the low income, poor and informal settlers living along river banks and low-lying coastal areas that are most vulnerable to rising tides and increasing water levels (Romualdez, 2013).
|Continent||Population density||Per capita GDP|
|North America||22.1||32 077|